Despite their reputation as all-consuming voids of darkness, it might come as a surprise to learn that black holes are responsible for the brightest known phenomena in the universe. This remarkable contrast is possible because of the violent forces that black holes generate, ripping apart all matter that approaches and turning gas clouds into searing beacons of light.
Sometimes, as shown in the animation below from NASA's Jet Propulsion Laboratory, these light shows can be on an order of magnitude that's difficult to comprehend. On July 31, 2019, NASA's Spitzer telescope captured an orbital clash between two black holes that generated an explosion of light brighter than that of a trillion stars or more than twice the brightness of our own Milky Way galaxy!
A hungry cosmic furnace
Black holes are capable of generating these light shows due to the way they wreak havoc on everything that dares come too close to their sphere of influence. As matter and gas swirls towards the black hole's center, it forms an accretion disc where particles heat up to millions of degrees. This ionized matter is then ejected as twin beams along the axis of rotation.
Depending on our perspective from Earth, the jets are either known as a quasar (viewed at an angle to Earth), a blazar (pointed directly at Earth), or a radio galaxy (viewed perpendicular to Earth). Either way, these light shows — which are the absolute brightest known — and their accompanying radio emissions help researchers discover new black holes that might otherwise go undetected.
Our own quiet giant
While most black holes are active enough to generate light across the electromagnetic spectrum, the supermassive one at the center of our own Milk Way is relatively quiet. Named Sagittarius A* and roughly 4 million times more massive than our own sun, researchers are attempting to figure out why this giant is something of a deep sleeper.
"As a black hole, as an energetic system, it's almost dead," Geoffrey Bower of the Academia Sinica Institute of Astronomy and Astrophysics in Hilo, Hawaii told Quanta Magazine.
Almost, but not quite. In May 2019, scientists observing Sagittarius A* in infrared at the WM Keck Observatory in Hawaii were surprised to see it generate an extremely luminous flare. You can see the time-lapse of the event below.
"The black hole was so bright I at first mistook it for the star S0-2, because I had never seen Sgr A* that bright," astronomer Tuan Do of the University of California Los Angeles told ScienceAlert. "Over the next few frames, though, it was clear the source was variable and had to be the black hole. I knew almost right away there was probably something interesting going on with the black hole."
While it's likely that the outburst was the result of Sagittarius A* coming in contact with a gas cloud or some other object, researchers are eager to learn more about both its feeding patterns and relative lack of general activity.
SOFIA may offer answers
Streamlines showing magnetic fields layered over a color image of the dusty ring around the Milky Way’s massive black hole. (Photo: Dust and magnetic fields: NASA/SOFIA; Star field image: NASA/Hubble Space Telescope)
One recent upgrade that may explain the relative quiet at the center of our galaxy is the new High-resolution Airborne Wideband Camera-Plus (HAWC+) that was added last summer to NASA's Stratospheric Observatory developed for Infrared Astronomy (SOFIA).
The HAWC+ is capable of measuring the powerful magnetic fields generated by black holes with extreme sensitivity. When it was pointed at Sagittarius A*, researchers discovered that the shape and power of its magnetic field is likely pushing gas into an orbit around it; therefore keeping the gas from feeding into its center and triggering a steady glow.
"The spiral shape of the magnetic field channels the gas into an orbit around the black hole," said Darren Dowell, a scientist at NASA's Jet Propulsion Laboratory, principal investigator for the HAWC+ instrument, and lead author of the study, said in a statement. "This could explain why our black hole is quiet while others are active."
Researchers hope instruments like HAWC+, as well as increased observations from the global Event Horizon Telescope (EHT), might help shed further light on one of our galaxy's most mysterious objects.
"This is one of the first instances where we can really see how magnetic fields and interstellar matter interact with each other," added Joan Schmelz, Universities Space Research Center astrophysicist at NASA Ames Research Center in California's Silicon Valley, and a co-author on a paper describing the observations. "HAWC+ is a game-changer."